**4. Precision polishing of SUS310S molding die**

In the slumping process, the use of stainless steel SUS310S as the molding die material is proposed because of its good heat resistance. In our previous study, a SUS304 stainless-steel molding die with nanoscale surface roughness was successfully prepared [13]. However, intergranular corrosions occurred at about 600°C on its surface after the slumping process, therefore, in this work, stainless steel SUS310S was selected because of its improved heat resistance [14, 15]. The temperature of 600°C is a softening one used to transform glass plates, in the case of using SUS304, these temperatures almost accords. Therefore, the SUS310S material that intergranular corrosion temperature is over 750°C was selected as an improvement research [16]. By this background, precision polishing experiments were conducted using various polishing pressures, polishing times, and surface roughnesses.

#### **4.1 Precision polishing experimental procedure**

**Table 1** summarizes the conditions used for the precision polishing experiments, and **Figure 3** presents an overview of the precision polishing experiment. The precision polishing was performed using a bench-type polishing machine (MA-200, Musashino Denshi). The end surfaces of the stainless steel SUS310S workpieces

#### **Figure 2.**

*Outline of the glass forming techniques "slumping method". (a) Preparation of molding die. (b) Processing set up. (c) Demolding process. (d) Deformation process.*


**Table 1.**

*Experiment conditions.*

**Figure 3.** *Overview of precision polishing machine and experiment.*

(50 mm diameter, 10 mm thickness) were polished, as shown in **Figure 4**. The polishing pad was a suede-type pad, and the polishing liquid was a 0.5% alumina (# 3000) mixture dispensed at a rate of 50 ml/h by a tube pump (PST110, Iwaki). The surface roughness of the workpieces and formed glass plates were measured using a surface roughness tester (SJ-201, Mitsutoyo) and 3D optical surface profiler (NewView7100, ZYGO). Measurements were taken approximately 5, 12, and 20 mm from the center of the workpiece for the "inside", "center", and "outside" positions, respectively.

## **4.2 Results of precision polishing for SUS310S**

**Figure 4** shows the workpieces before and after polishing. Cutting grooves were apparent on the workpiece surface before polishing, as shown in **Figure 4(a)**; these grooves were formed over the entire surface by the previous cutting process. Generally, it has been found that a machining for stainless steels called "hard to cut materials" is difficult to make smooth surface, because diamond bites cannot be used in ultra-precision cutting process [17, 18]. However, after 37 h of polishing, the workpiece had a mirror surface with the grid pattern of the under sheet, as observed in **Figure 4(b)**. Comparison of the surfaces reveals that the precision polishing process reduced the surface roughness.

*Precision Polishing Techniques for Metal Molding Dies and Glass Forming Technology… DOI: http://dx.doi.org/10.5772/intechopen.99208*

#### **Figure 4.**

*Comparison of before and after polishing workpieces. [SUS310S, 0.5%-Al2O3 polishing liquid, 1.7 kPa]. (a) before polishing. (b) after polishing.*

**Figure 5** presents shows the relationship between the surface roughness *Rz* of the polished SUS310S surface and polishing time for a polishing pressure of 1.7 kPa. When the precision polishing process was performed for 1 hr., the surface roughness of the outside region was the smallest of all the regions with *Ra* = 0.07 μm and *Rz* = 0.64 μm, as shown in **Figure 5(b)**. After more than 2 hrs of polishing, repeated small variations in the surface roughness were observed and the surface roughness was reduced equally over the entire surface. After approximately 15 hrs of precision polishing, the surface roughness was almost constant at *Ra* = 0.03 μm and *Rz* = 0.25 μm. After that, the surface roughness gradually decreased, and finally, a surface roughness of *Ra* = 0.02 μm and *Rz* = 0.16 μm was achieved after 37 hrs of precision polishing.

In our previous study on precision polishing with a polishing pressure of 0.7 kPa (only the pressure of the die weights without additional pressure), the surface roughness of the outside region was less than 1.0 μm after 5 hrs of polishing [12, 13]. However, in the current study using a pressure of 1.7 kPa, the surface roughness was smaller than *Rz* = 1.0 μm after less than 2 hrs; these results indicate that the polishing efficiency was improved with higher polishing pressure.

**Figure 6** compares the surface roughness (a) before polishing, (b) after 1 h polishing, and (c) after 37 hrs polishing for a polishing pressure of 1.7 kPa. **Figure 6(a)** confirms that the tool feed interval roughness resulted in the cutting grooves observed in **Figure 4(a)**, for which the surface roughness was *Ra* = 0.4 μm and *Rz* = 2.7 μm. After 1 h polishing, the surface roughness decreased (*Ra* = 0.07 μm, *Rz* = 0.66 μm), as observed in **Figure 6(b)**. Finally, after 37 h polishing, the cutting grooves were no longer observed, as shown in **Figure 6(b)**, and the surface roughness was *Ra* = 0.01 μm and *Rz* = 0.10 μm.

To examine the precision polished surface more closely and precisely, the precision polished workpiece in **Figure 6(c)** was characterized using an optical surface profiler. The polished surface had an arithmetic mean roughness of *Sa* = 1.8 nm and a maximum height roughness *Sz* = 20.4 nm in the 3D area. The surface roughness in the 2D measurement is *Ra* = 1.7 nm and *Rz* = 9.1 nm, with some small undulations observed. The objective of this study was to achieve a surface

**Figure 5.**

*Surface roughness of polished SUS310S surface. [SUS310S, 0.5%-Al2O3 polishing liquid, 1.7 kPa]. (a) inside of workpiece (r = 5 mm). (b) outside of workpiece (r = 20 mm).*

roughness of less than *Ra* = 2 nm, which would enable the use of the SUS310S stainless steel molding die.

Our previous polishing experiments on stainless steels with a polishing pressure of 1.7 kPa used SUS310S. Therefore, the SUS304 and SUS310S molding die polishing results are compared for a polishing pressure of 1.2 kPa [12]. The final polished surface roughness was in the same range of *Rz* = 10 nm. The times required to reduce the surface roughness *Rz* from 4 to 1 μm for SUS304 and SUS310S were 7 and 2 h, respectively; for a polishing pressure of 0.7 kPa, these values were 18 and 5 hrs, respectively. These results indicate that the SUS310S molding die may be easier to polish than the SUS304 molding die.

### **4.3 Designing and manufacturing "centerless polishing machine" for cylindrical molding dies**

**Figure 7** shows the designed and manufactured centerless polishing machine images to polish cylindrical molding dies. This molding die polishing system will be *Precision Polishing Techniques for Metal Molding Dies and Glass Forming Technology… DOI: http://dx.doi.org/10.5772/intechopen.99208*

**Figure 6.** *Surface roughness of polished SUS310S surface. (a) before polishing [Ra = 0.40* μ*m, Rz = 2.70* μ*m]. (b) after 1 hour polishing [Ra = 0.07* μ*m, Rz = 0.66* μ*m]. (c) after 37 hours polishing [Ra = 0.02* μ*m, Rz = 0.15* μ*m].*

used for making large scale glass mirrors that are over 600 mm diameter in X-ray telescope field. Therefore large size molding dies have to be manufactured with very small surface roughness. From these background, "centerless polishing machine" was designed. The direction of set-up and polishing of workpieces are lateral direction as shown in **Figure 7(a)**, because it is easy to support and rotate without center fix system of heavy workpieces. In addition, to polish cylindrical molding die surface, polishing pressure that shown in follow polishing examinations for edge surface acts uniformly by using polishing parts self-weight and gravity as shown in **Figure 7(b)**. In these images, a workpiece size is 50 mm diameter and 60 mm length, however the
